1. Field of the Invention
The present invention relates to the conversion of calcium compounds into solid and gaseous materials. In one preferred aspect, the present invention relates to the conversion of calcium compounds into a gaseous product and a solid cement clinker product. In another preferred aspect, it relates to the conversion of calcium phosphate materials into phosphorus and its compounds.
2. Description of the Prior Art
Numerous processes for converting calcium mineral compounds into useful products have been proposed. For example, the use of sulfuric acid to decompose phosphate rock into gypsum and phosphoric acid is a widely practiced technique. The conversion of phosphate rock into phosphorus and a calcium silicate slag is practiced to produce phosphorus for uses requiring high purity. Another widely practiced technique is the conversion of calcium carbonate and clay minerals into a cement clinker and carbon dioxide in a rotary kiln. These processes suffer from the deficiency of either consuming large amounts of energy, producing objectionable by-products, or both.
The production of wet process phosphoric acid by the digestion of phosphate rock with sulfuric acid produces a particularly troublesome waste product, phosphogypsum. No suitable means for disposing of the phosphogypsum or converting it to useful products have to date been developed. Some countries dispose of phosphogypsum by dumping it in the ocean. However, in the United States, the phosphogypsum has usually been accumulated in large piles which have associated ponds holding large quantities of water.
Phosphogypsum represents a potential source of significant quantities of calcium and sulfur. Thus, a need exists for an economical process which can convert phosphogypsum into useful calcium and sulfur products.
It would also be desirable to develop a new technique for directly converting phosphate rock into phosphoric acid inexpensively so as to eliminate the wet process technique. If the wet process technique could be replace, the phosphogypsum problem and the need for large sulfuric acid plants would be eliminated.
The only technique which has successfully been suggested as an alternative to the wet process technique is the electric furnace technique. This process produces elemental phosphorus which may then be oxidized and hydrated to produce phosphoric acid. However, phosphoric acid produced using the electric furnace process, while extremely pure, is too expensive to be utilized on a large scale in the fertilizer industry, the major world consumer of phosphoric acid. Thus, the electric furnace acid has found use only in those applications where high purity is required, such as foods, drugs or cosmetics.
The first documented attempts to convert calcium sulfate to sulfur dioxide and cement clinker occurred during World War I in Germany. W. S. Mueller investigated the decomposition of calcium sulfate (anhydrite) and together with H. Kuhne developed a rotary kiln technique to convert calcium sulfate into sulfur dioxide and cement clinker. Subsequently, other plants were built to practice this technique in locations where alternative sources of sulfur were either not available or excessively priced. Hull et al in Industrial and Engineering Chemistry, Volume 49, No. 8, August, 1957, pages 1204-1214 contains a summary of the efforts which have been made to convert anhydrite into cement clinker and sulfur dioxide.
Subsequently, with the increased availability of phosphogypsum, it was natural to suggest utilizing phosphogypsum in place of the anhydrite which had previously been utilized. These processes generally involved treating the phosphogypsum to reduce its phosphorus and fluorine content prior to the reaction with silica and other cement forming elements in the rotary kiln. This pretreatment was necessary since phosphorus and fluorine present in the phosphogypsum would result in excessive quantities of these elements being incorporated in the cement clinker product, yielding an unacceptable cement. Although numerous clean-up processes have been developed, none have achieved status as an accepted practice since all such processes have proven too expensive. Thus, this process has been limited to the utilization of comparatively clean phosphogypsum in which the presence of fluorine, phosphorus and other impurities is minimized. However, such processing is cost prohibitive in most cases and little used.
The rotary kiln technique for converting phosphogypsum into cement and sulfur dioxide is described in the article: "Manufacture of Cement from Industrial By-Products," Chemistry and Industry, February, 1971. Production of sulfuric acid and cement from phosphogypsum using the "OSW process" is described in: Chemical Age of India, Volume 27, No. 12, December, 1976 and "Getting Rid of Phosphogypsum - II", Phosphorus and Potassium, No. 89, May/June, 1977. The rotary kiln techniques all utilize a reducing zone followed by an oxidizing zone.
The OSW process, which is typical of the rotary kiln processes, has a residence time of over six hours in the main kiln. These kilns operate at temperatures under 2900 degrees Fahrenheit, typically about 2700 degrees Fahrenheit. These processes consume about 21 million BTU per ton of cement clinker product. The phosphogypsum raw material must contain 0.5% (W/W) or less of P.sub.2 O.sub.5 and 0.15% (W/W) or less of fluorine to produce an acceptable clinker product. The raw materials are usually pelletized prior to processing. Kiln gases typically contain 9-12% SO.sub.2 (dry basis, by volume), but when mixed with sufficient air to oxidize the sulfur dioxide to sulfur trioxide in sulfuric acid manufacture, this stream is diluted to 4-5% SO.sub.2. This low concentration of sulfur dioxide requires that larger than normal vessels and auxiliary equipment be employed in sulfuric acid manufacture. This limits the rotary kiln technique to sulfuric acid plants specifically designed to utilize the product gas stream from the rotary kilm. Hence, the rotary kiln technique is limited by requirements of large equipment, high phosphogypsum purity, low energy efficiency and low SO.sub.2 product gas strength.
Dr. T. D. Wheelock of Iowa State University studied the technique of decomposing phosphogypsum in a fluidized bed to produce sulfur dioxide and a quick lime product as described in U.S. Pat. Nos. 3,087,790; 3,260,035; 3,607,045; and 4,102,989. As in the case of the rotary kiln technique, the Iowa State technique involves a reducing zone and an oxidizing zone in the reactor. The Iowa State technique produces a sulfur dioxide which can be converted into sulfuric acid, but the quick lime it produces is very impure, thus having little, if any, market value. For decomposition of phosphogypsum processes to be economical, the calcium by-products must be pure enough to have a good market value. Thus, the Iowa State technique is not economically attractive.
It has also been proposed, as by Jonasson et al, World Cement, December, 1982, pages 383-388, to produce Portland cement clinker from phosphogypsum in an electric arc furnace. While it is entirely probable that phosphogypsum could be converted into cement clinker and sulfur dioxide in an electric furnace, such a process would be very expensive since large amounts of electric energy would be consumed.
The commercial production of phosphorus is performed in electric arc furnaces. Typical residence times in these furnaces range from four to nine hours operating at temperatures from 2250 to 2650 degrees Fahrenheit. Typical furnaces consume about 12,000 kw-hr. per ton of phosphorus product. The reactants (including phosphate rock, high quality coke, and silica) are usually agglomerated by pelletization prior to introduction into the furnace. Often the phosphate rock and silica require calcination prior to pelletization. In addition to the product phosphorus withdrawn in the process gases, the electric furnace process produces calcium silicate and ferrophosphorus slags which have little, if any, value. Normally, these slags are placed in large slag piles at the production site.
In the past, phosphorus has been produced by blast furnace technology. This process consumed about 2.4 tons of coke per ton of phosphorus produced. The process gas stream contained a very low concentration of phosphorus, and, thus, the process required large vessels and large capital expenditures. Other operating parameters were very similar to those for the electric furnace.
The electric furnace and blast furnace processes both suffer from requirements for large quantities of energy, large equipment and creation of large slag piles. A further problem is that when the desired product is a phosphorus oxide, these processes require that energy be supplied to reduce the phosphate in the ore to elemental phosphorus and that subsequently the phosphorus be oxidized to the desired oxide form. This requires an additional step with considerable loss of energy.
Several researchers have attempted to reduce the energy requirements for phosphorus production by reducing phosphate rock in high temperature plasmas. Chase et al in "Plasma Jet Process for Making Elemental Phosphorus," Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 2, 1979, pages 261-266, describe a process by which phosphate rock is reduced with liberation of phosphorus in a high temperature plasma. Chase et al were able to obtain their highest yield of eighty-one percent at a calculated reactor inlet temperature of 5525 degrees Fahrenheit with a gas residence time (approximately the same as the solids) of 0.21 seconds. At an inlet temperature of 4445 degrees Fahrenheit and an 0.38 second residence time, the yield fell to 56 percent. However, due to the inherent large thermal graidents in plasma chemical reactors, the solids undoubtedly never reached the reactor inlet temperature. Although such processes effectively reduce phosphate ores at temperatures in excess of 5000 degrees Fahrenheit in very short residence times, the large electrical power consumption required to heat the plasma gas makes such processes uneconomical.
In U.S. Pat. No. 3,481,706, Veltman et al disclose a process by which phosphorus is produced by reducing phosphate rock in a flame. In this process, finely divided preheated phosphate rock and a hydrocarbon or carbon reductant was passed through a flame generated by combustion of a hydrocarbon or coal with oxygen. The flame temperature ranged from 3000 degrees Fahrenheit to 4500 degrees Fahrenheit. Silica could be added to produce a calcium silicate slag if that was the desired calcium product. The phosphate ore was then allowed to fall freely through a reactor one hundred feet in height followed by separation of the gaseous and liquid phases. This reactor was manufactured from refractory lined carbon steel. To avoid excessive cooling throughout the reactor, further heating could be accomplished along the vertical reactor by positioning further burners or by electric discharge on the gas flame. If the flame was augmented by an electric discharge, it was necessary to add an ionizable salt with the phosphate rock. Veltman et al noted, but made no pertinent claim to, the observation "that under certain process conditions, and for reasons not understood, phosphorus oxides may be concurrently produced." These reasons will become more clear as the current invention is revealed, but Veltman et al at best produced very small quantities of phosphorus oxides.
Those skilled in the art recognize that the process of Veltman et al suffers from many deficiencies such as lack of provision for avoiding or removing the massive buildup of solid and liquid products on the reactor walls resulting in excessive heat loss through the walls as well as damage to the refractory wall, and insufficient heating of phosphate ore as it passes through the flame, resulting in low conversion and inefficient use of energy. The result was that the overall temperature of material exiting the reactor was only 3900 degrees Fahrenheit, even with the additional heat input from the additional burners. At 3900 degrees Fahrenheit, the reaction kinetics are slow enough that Veltman et al were unable to obtain complete reaction even during the residence time resulting from a one hundred foot free fall through the reactor.
Attempts have also been made to produce phosphorus by the reduction of phosphate ore in a rotary kiln such as described by Lapple in U.S. Pat. Nos. 3,235,330 and 3,241,914. These processes have been known for some time but have not been practiced because of poor phosphorus yield. In other rotary kiln processes, such as that described by Megy et al in U.S. Pat. No. 4,351,813, phosphorus is released from a bed in a rotary kiln under reducing conditions. In an oxidizing zone over the bed, the phosphorus may be burned to phosphorus oxides with the release of radiant energy which is absorbed by the bed, providing a source of heat for the reduction reaction step. Whereas previous rotary kiln processes for phosphorus production had suffered from problems of premature carbon burnout, excessive liquid phase formation at higher temperatures and excessively slow reaction rates at lower temperatures, Megy et al avoided many of these problems by purging the bed with an inert gas. The use of inert gas allowed Megy et al to avoid excessive liquid phase and to offer improved performance by operating at higher temperatures. Although this process gave excellent yields of either phosphorus or phosphorus oxides, the process suffered from many deficiencies such as requirements for large volumes of purge gas and incomplete combustion of carbonaceous material. Further, because of the formation of excessive liquid phase at temperatures exceeding 2700 degrees Fahrenheit, the reaction kinetics were still so slow as to require large rotary kilns to perform the reaction.
Another approach to the thermal decomposition of phosphate rock has been described by A. L. Mosse et al in "Production of Phosphorus - Containing Compounds in Plasmachemical Reactors When Processing Fine-Dispersed Natural Phosphates," 2nd International Congress of Phosphorus Compounds Proceedings, Institut Mondial du Phosphate, 1980. Mosse et al studied the reaction: EQU 2Ca.sub.3 (PO.sub.4).sub.2 =6CaO+P.sub.4 O.sub.10
in a gas plasma. Mosse et al were able to obtain P.sub.4 O.sub.10 at temperatures as low as 4400 degrees Fahrenheit. Although Mosse et al found good yields, their work was limited by the basic deficiencies of plasma reactors in that they had insufficient residence times, thus requiring excessive temperatures and the energy consumption, due to the heating requirements of the large volume of plasma gas, was excessive. Further, the very short residence time and high degree of dilution by the plasma gas did not allow study of the energetically more favorable reaction: EQU 2Ca.sub.3 (PO.sub.4).sub.2 +6SiO.sub.2 =6CaO.SiO.sub.2 +P.sub.4 O.sub.10.
This reaction is favorable at temperatures as low as 2800 degrees Fahrenheit.
Thus, there exists a need for a technique which economically converts calcium minerals into useful solid and gaseous products. In particular, a need continues to exist for an improved method for converting calcium minerals into cement clinker and gaseous products.
A further need exists for a technique for converting phosphogypsum into sulfur dioxide and a useful solid calcium product. In particular, for a technique for converting phosphogypsum into a concentrated sulfur dioxide stream and Portland cement clinker.
Still a further need exists for an economical technique for converting phosphate rock into phosphorus and calcium oxide, calcium silicate or Portland cement clinker.
Still a further need exists for an economical technique for converting phosphate rock into phosphorus oxide and calcium oxide, calcium silicate or Portland cement clinker.